This application is based on and incorporates herein by reference Japanese Patent Applications No. 2000-261280 filed Aug. 30, 2000 and No. 2001-139993 filed May 10, 2001.
The present invention relates to a current supply circuit for starters which drive engines.
In conventional vehicles, a starter drives an engine until the engine operates with air-fuel mixture combustion. The engine operation is maintained in the idling condition when the vehicle is at rest at traffic lights. An economy running system is increasingly used in recent years to stop the engine when the vehicle is at rest at the traffic lights, so that fuel economy and exhaust emission are improved.
In the economy running system, a radio and other electric devices are very often in operation when the starter automatically re-starts the engine in the vehicle rest condition at traffic lights. The starter is subjected to a large load at the initial stage of driving the starter, because the engine has a large stationary inertia. The current increases excessively as shown by the solid line A in FIG. 2. As a result, when the starter is re-started from the vehicle rest condition, an excessive current flows at the initial stage and influence the radio and other electric devices in operation at that time.
Further, in case a starter is designed to operate with higher voltage, the number of turns of the pull-in coil of a magnet switch, which controls the current supply to the starter motor, increases resulting in an increase in the inductance of the pull-in coil of the magnet switch. This causes delay in turn-on time of the magnet switch.
When the magnet switch turns on, a movable contact crashes a fixed contact and bounces. However, the bouncing occurs with a time delay due to increase in the inductance of the pull-in coil of the magnet switch. If the starter is not a high voltage type, the inductance of the pull-in coil of the magnet switch is low. As a result, the magnet switch turns on quickly and bouncing occurs at position xcex1 in FIG. 3. However, if the starter is designed to operate with high voltage, the turn-on time of the magnet switch delays and bouncing occurs at position xcex2 in FIG. 3.
When the time of bounce delays as shown by xcex2, the magnet switch turns on and off the large current by bouncing causing generation of electric arc at contact terminals of the magnet switch. This arc generates noise and causes melting of the contact terminals, resulting in low reliability of the magnet switch.
It is therefore an object of the present invention to reduce a current supplied when driving a starter is initiated, so that reliability of a magnet switch is improved and noise generated upon bouncing is reduced.
According to the present invention, a current supply circuit for starters comprises a starter motor for starting an engine, a magnet switch for turning on and off a current supplied to the starter motor, and a rise slow-down device for slowing down a speed of rise of the current supplied to the starter motor.
The rise slow-down device may be an inductance coil connected in series with the starter motor and slows down the speed of rise of the current by inductance of the inductance coil. The rise slow-down device may alternatively a capacitor connected in parallel with the starter motor and slows down the speed of rise of the current by capacitance of the capacitor. Preferably, a time period required for the current to attain a peak when the current supplied to the starter motor is slowed down satisfies Txe2x89xa6(10/12)V, in which T indicates the time period and V indicating a driving voltage for the starter motor.